The present invention relates to methods of using the three-dimensional quadrupole ion trap mass spectrometer ("ion trap") which was initially described by Paul, et al.; see, U.S. Pat. No. 2,939,952. In recent years, use of the ion trap mass spectrometer has grown dramatically, in part due to its relatively low cost, ease of manufacture, and its unique ability to store ions over a large range of masses for relatively long periods of time. This latter feature makes the ion trap especially useful in isolating and manipulating individual ion species, as in a so-called tandem MS or "MS/MS" or MS.sup.n experiment where a "parent" ion species is isolated and fragmented or dissociated to create "daughter" ions, which may then be identified using traditional ion trap detection methods or further fragmented to create granddaughter ions, etc.
Isolation of individual ion species also has importance in other applications beside isolation of parent ions for MS/MS experiments. Given the relatively low cost and sensitivity of present-day commercial ion traps, they can be used to monitor for the presence of specific compounds or groups of related compounds, e.g., monitoring for the release of toxic gases in an production area. Controlling an ion trap to selectively isolate specific ion species of interest can be used to optimize the sensitivity of the trap for the selected species, which otherwise would be poorly detectable or completely undetectable. In this regard, it is noted that one of the drawbacks of the ion trap is its limited dynamic range and sensitivity to the space charge created by the ions trapped within the device. Thus, the presence of a substantial number of ions in the trap, other than the ions of interest, can substantially degrade the sensitivity of the trap to the ions of interest. In order to optimize sensitivity to the ions of interest, it is best to rid the trap of the other ion masses.
The quadrupole ion trap comprises a ring-shaped electrode and two end cap electrodes. Ideally, both the ring electrode and the end cap electrodes have hyperbolic surfaces that are coaxially aligned and symmetrically spaced. By placing a combination of AC and DC voltages (conventionally designated "V" and "U", respectively) on these electrodes, a quadrupole trapping field is created. A trapping field may be simply created by applying a fixed frequency (conventionally designated "f") AC voltage between the ring electrode and the end caps to create a quadrupole trapping field. The use of an additional DC voltage is optional, and in commercial embodiments of the ion trap a DC trapping voltage is not normally used. It is well known that by using an AC voltage of proper frequency and amplitude, a wide range of masses can be simultaneously trapped.
The mathematics of the quadrupole trapping field created by the ion trap are well known and were described in the original Paul, et al., patent. For a trap having a ring electrode of a given equatorial radius r.sub.0, with end cap electrodes displaced from the origin at the center of the trap along the axial line r=0 by a distance z.sub.0, and for given values of U, V and f, whether an ion of mass-to-charge ratio (m/e, also frequently designated m/z) will be trapped depends on the solution to the following two equations: ##EQU1## where .omega. is equal to 2.pi.f.
Solving these equations yields values of a.sub.Z and q.sub.Z for a given ion species having the selected m/e. If the point (a.sub.Z, q.sub.Z) maps inside the well-known stability envelop for the ion trap, the ion will be trapped by the quadrupole field. If the point (a.sub.Z, q.sub.Z) falls outside the stability envelop, the ion will not be trapped and any such ions that are introduced within the ion trap will quickly move out of the trap. By changing the values of U, V or f one can affect the stability of a particular ion species. Note that from Eq. 1, when U=0, (i.e., when no DC voltage is applied to the trap), a.sub.Z =0.
(It is common in the field to speak of the "mass" of an ion as shorthand for its mass-to-charge ratio. As a practical matter, most of the ions in an ion trap are singly ionized, such that the mass-to-charge ratio is the same as the mass. For convenience, this specification adopts the common practice, and generally uses the term "mass" as shorthand to mean mass-to-charge ratio.)
Each ion in the trapping field has a "secular" frequency which depends on the mass of the ion and on the trapping field parameters. As is well-known, it is possible to excite ions of a given mass that are stably held by the trapping field by applying a supplemental dipole voltage to the ion trap having a frequency equal to the secular frequency of the ion mass. Ions in the trap can be made to resonantly absorb energy in this manner. At relatively low voltages, a supplemental dipole voltage can be used to cause ions of a specific mass to resonate within the trap, undergoing dissociating collisions within molecules of a background gas in the process. This technique, called collision induced dissociation or "CID," is commonly used in MS/MS to dissociate parent ions to create daughter ions. At higher voltages, sufficient energy is imparted by the supplemental voltage to cause ions having a secular frequency matching the frequency of the supplemental voltage to leave the trap volume. This technique is now commonly used to eliminate unwanted ions from the ion trap, and to eject ions from the trap for detection by an external detector.
The typical basic method of using an ion trap consists of applying an rf trapping voltage (V.sub.0) to the trap electrodes to establish a trapping field which will retain ions over a wide mass range, introducing a sample into the ion trap, ionizing the sample, and then scanning the contents of the trap so that the ions stored in the trap are ejected and detected in order of increasing mass. Typically, ions are ejected through perforations in one of the end cap electrodes and are detected with an electron multiplier. More elaborate experiments, such as MS/MS, generally build upon this basic technique, and often require the isolation of specific ion masses, or ranges of ion masses in the ion trap.
Once the ions are formed and stored in the trap a number of techniques are available for isolating specific ions of interest. It is well-known that when the trapping field includes a DC component, the trapping field parameters (i.e., U, V and f) can be adjusted to isolate a single ion species, or a very narrow mass range, in the trap. A problem with this approach is that it is difficult to control the trapping field parameters with the high degree of precision, and it is difficult to calculate the precise combination of trapping field parameters needed to isolate a single mass or a narrow range of masses. Another problem is that most commercial ion traps do not have the ability to apply a DC trapping voltage, and adding this capability increases the amount and cost of the system hardware that is required. Moreover, it is noted that this method cannot be used to isolate multiple discontinuous masses. Finally, it is noted that the ions to be retained in the field will be near the edge of the stability boundary such that the trapping efficiency is not optimal, and may be rather poor.
U.S. Pat. No. 4,736,101 describes another method of isolating an ion for MS/MS experiments. According to the technique taught by the '101 patent, a trapping field is established to trap ions having masses over a wide range. This is done in a conventional manner, as was well known in the art. Next, the trapping field is changed to eliminate ions other than the selected ion of interest. To do this the rf trapping voltage applied to the ion trap is ramped so as to cause ions of low mass to sequentially become unstable and be eliminated from the trap. The ramping of the rf trapping voltage is stopped at the point at which the mass just below the ion of interest is eliminated from the ion trap. The '101 patent does not teach how to manipulate the trapping field to eliminate ions having a mass that is higher than the mass of interest when no DC trapping voltage is applied. After the contents of the ion trap have been limited by the foregoing technique of changing the trapping voltage, the trapping voltage is relaxed so that, once again, ions over a broad range are trapped. Next, the parent ions within the ion trap are dissociated, preferably using CID, to form daughter ions. Finally, the ion trap is scanned by again ramping the quadrupole trapping voltage so that ions over the entire mass range sequentially become unstable and leave the trap.
The major deficiency of the method of the '101 patent is its failure to teach how to eliminate high mass ions from the trap without using a trapping field having a DC component. In addition, the technique of causing the low mass ions to be eliminated from the ion trap by instability scanning is also problematic. If m.sub.P is the mass to be retained in the trap, and the trapping field is manipulated to cause m.sub.P-1 to become unstable, then m.sub.P will, at that point, be very close to the stability boundary. Again, this may cause the trapping efficiency for m.sub.P to be quite low, and requires precise control of the trapping voltage as it is ramped to eliminate unwanted low mass ions.
Another method of isolating an individual ion species in an ion trap is described in U.S. Pat. No. 5,198,665 (the '665 patent) issued to one of the present inventors and coassigned herewith. (The disclosure of the '665 patent is hereby incorporated by reference.) According to the '665 patent, masses lower than the mass to be retained (m.sub.P) are first sequentially scanned out of the trap using resonance ejection. This has the advantage that m.sub.P-1 can be eliminated from the trap while m.sub.P is far from the stability boundary. After the low mass ions are so eliminated, a broadband supplemental signal is applied to the trap to eliminate the higher mass ions. The trapping voltage may be reduced slightly while applying the supplemental broadband voltage to bring ions just above m.sub.P into resonance. While this technique is capable of producing highly accurate results, it is somewhat complex and cannot be used to isolate multiple discontinuous masses from the ion trap. In addition, since high mass ions remain in the trap while the low mass ions are being eliminated, a significant space charge remains. Unless proper measures are taken, this space charge can interfere with the accuracy of experiments using the technique.
It is also known in the prior art to apply various types of supplemental broadband voltage signals to the ion trap to simultaneously eliminate multiple unwanted ion species from the trap. The prior art generally teaches use of (1) broadband signals that are constructed from discrete frequency components corresponding to the resonant frequencies of the unwanted ions; and (2) broadband noise signals that essentially contain all frequencies, such that they act on the entire mass spectrum, and which are filtered to remove frequency components corresponding to the secular frequency(ies) of the ions that are to be retained in the ion trap. In all of the known prior art methods, the trapping field is held constant while the supplemental broadband voltage is applied to the ion trap.
According to these prior art methods, in order to retain an single ion species in an ion trap, it is necessary to apply a supplemental voltage waveform which has a very large number of frequency components so that the waveform will excite all of the ions which may potentially be held in the trapping field, other than the ion mass(es) of interest. A typical ion trap sold by the assignee of the present invention covers a mass range of about 50-650 amu under normal trapping conditions. If, for the sake of discussion, we assume that there is a single frequency component required to excite each integer ion mass, then approximately 600 frequency components would be required to resonantly eject the entire mass spectrum. However, this number of frequency components would only excite ions having integer masses. If ions were present in the trap having multiple charge, (e.g., a doubly ionized molecule), the resulting value of the mass-to-charge ratio may not be an integer value. In addition, it is known that space charge in the trap can affect the secular frequency of the trapped ions, such that a frequency component, included in a supplemental waveform to excite a particular ion mass, would not work. Thus, as a practical matter, when using the prior art techniques to isolate a single ion mass, or a narrow range of ion masses, in an ion trap, there is a need to include a much larger number of frequency components.
For example, U.S. Pat. No. 5,256,875, suggests that thousands of frequency components should be used. The patent notes that the frequency spacing in the broadband excitation signal should be sufficiently small that the signal presents a substantially continuous band of frequencies to the physical system, and goes on to state that the width of a "notch" in the spectrum designed to allow a single ion mass to be retained in the trap, should be substantially less than 500 Hz at the low frequency end of the spectrum. This, in turn, requires that the frequency spacing in the areas on either side of the notch be even narrower. As a practical matter, however, this is not workable since it does not account for the fact that the secular frequency of ions in the trap varies with the space charge in the trap. As described below, the resonance width of ions can be substantially more than 500 Hz.
Neither the '875 patent, nor the other patents which teach the use of broadband excitation signals to eliminate en masse unwanted ions from the ion trap, adequately address the fact that the spacing of the secular frequencies of adjacent ion masses varies across the mass spectrum. For low masses, the secular frequencies of adjacent integer masses are far apart, whereas at high masses they are quite close. As a result, at low masses, if the ion of interest is not an integer mass, or if space charge or trapping field irregularities have caused a shift in the nominal secular frequency, there is a risk that the mass will not be excited and eliminated. On the other hand, in the high mass range, a single frequency component may cause resonance of multiple mass values, in which case a narrow "notch" in the broadband signal might not be sufficient to ensure that a desired ion will be retained in the ion trap.
A disadvantage of the prior art, which relies on waveforms containing a very large number of frequency components, is the high power requirements associated with having each of the frequency components present at sufficiently high voltage levels to cause excitation of ions across the mass spectrum. This disadvantage exists both for noise signals and for constructed waveforms, i.e., waveforms in which the frequency components are predetermined either by direct frequency selection or by an algorithm, such as an inverse Fourier transform of a frequency domain excitation spectrum to create a time domain excitation waveform. In a constructed waveform, it is important to further control the phases of the frequency components to minimize the dynamic range of the excitation waveform. As the number of frequency components increases, the need for more elegant and time-consuming are needed to create a time domain signal with a reasonable dynamic range, i.e., a minimized peak-to-peak voltage. For example, the '875 patent teaches a rather complex and time-consuming iterative technique for generating a supplemental voltage waveform.
A further disadvantage of the prior art methods of using broadband signals to eliminate unwanted ions from an ion trap is the failure to address the fact that the resonance frequency and resonance width of the ions in the trap changes with the space charge in the trap and with the location that the trapped ions occupy in the trap.